How long does it take to react in a given
situation?

It depends on the stimulus, the circumstances and the
individual involved.

Here are some general guidelines:

 Imagine a simple experiment as might be done in a
psychology test lab: "See the light, hit the
button." The average reaction time in an experiment like
this is likely to be about 1/4 second (0.25 sec.). But times
will very, generally as we get older, for example, we get
slower.

 Suppose a child darts out in front of us while we
are driving down the street. Here the identification -
reaction time might be as little as 0.25 sec., but the
vehicle response time usually will be on the order of 0.75
sec. This is the time required for the driver to see the
stimulus, react to it by hitting the brakes and the time
required for the brakes to start to stop the vehicle. This is
a value often used by accident reconstructionists but it is
frequently optimistic. A 1 sec. vehicle response time -
another commonly used value - may be more realistic.

 Times for the above circumstances might be as high
as 2 sec. for an older driver under poor lighting conditions.
More complicated stimuli or environmental circumstances might
make the whole response interval even longer. It is important
to remember that the vehicle will be moving at the original
speed before the brakes take hold. -1.47 feet per second./
mile/hour.